The girl was found in the car park by the Paradisbukta swimming spot in Steinkjer in central Norway on Thursday evening, with police receiving a call at around 8.30pm.
"She was screaming hysterically when she was discovered. She was locked in the car with all the windows closed," Lars Erik Besseberg at Nord Trøndelag police told the adressa.no newssite.
"When she came out she was totally drenched with sweat and very warm," he added.
Police estimated that the child had been left in the car for about an hour.
The girl was discovered by passers-by who heard her screams and approached the car to investigate. They managed to cajole the 3-year-old into unlocking the car door and secure her release.
The girl remained in the care of the passers-by until the police arrived, eventually followed by the infant's parents.
The parents are reported to be in their late-20s and from the Steinkjer area and stand to face a criminal investigation.
"What they did was indefensible. Children should not be left alone in the car in the first place, especially not with all the windows closed. On a day like this a car becomes like a small sauna," Lars Erik Besseberg said to adressa.no.